This invention relates to printers for computer systems and more particularly to Cathode Ray Tube (CRT) exposure systems within such printers. Even more particularly, the invention relates to using a linear digital exposure engine to expose photosensitive cut-sheet media.
Digital photographic process printers that utilize cathode ray tubes (CRT""s) for media exposure typically expose the photographic media line by line directly from the face of the CRT or through fiber optics or through a Graded Index Lens. That is, the light from each element, or pixel, on the face of the CRT is conducted to the media directly, through a conventional lens, through a fiber optic face plate or through the graded index lens. The interior surface of the face plate typically has three phosphor stripes or bands; one for red, one for green, and one for blue, for the three color components that make up the elements. The controller for the CRT uses linear deflection amplifiers to select a phosphor and sweep an electron beam across one phosphor on the face of the CRT to expose one line on the media to one color at a time. This exposes the media in a deflection cycle that typically consists of a sweep, retrace, and hold period. During the sweep period, the CRT electron beam is unblanked and the beam is driven horizontally at a given vertical position while being intensity modulated to expose the media to the correct amount light at each location on the line of the media. The particular color being exposed is determined by the vertical position of the electron beam on the face of the CRT. To correctly expose each line of the media to each color, the media is moved as each color is selected and exposed.
In prior art systems that use rolls of paper, as shown in FIG. 1, moving the media comprised unrolling the paper from a supply roll 120 and pushing the paper with rollers 122 and 124 to expose each line of the print media to the CRT at each position and then cutting the paper into sheets by a cutting device 126 after the paper is exposed by CRT 100 and graded index lens 114. Because the roll is continuous, the entire print sheet is easily exposed since it is not cut by cutting device 126 until after the exposure. However, precise paper positioning is required to avoid cutting the page incorrectly such that a cut sheet contains a segment of unexposed media or that a segment of exposed media is left for the next sheet. Thus, a disadvantage of this approach is that the junction between adjacent prints must be accurately sensed and the cut precisely positioned in order to avoid these problems. Alternatively, a slug may be removed between images by using a double cut, but this causes paper waste and requires a mechanism for disposing of the slugs.
In prior art systems that use a take-up roller, as shown in FIG. 2, moving the media comprises unrolling the media from a supply roll 202 and pulling the paper past the print engine 100 with a take-up roller 206. Again the roll of media is continuous to each print is easily exposed since it is not cut until a later time after exposure and development.
If the print media is pre-cut before being inserted into the printing mechanism, however, a problem arises with exposing the leading and trailing edges of the paper. Since these edges are typically gripped by some mechanism to hold the paper, this gripping mechanism covers the edges, thus preventing exposure of the edges.
There is a need in the art for an apparatus and method for exposing an entire sheet of cut-sheet print media. There is another need in the art for such a system that can print while moving the print media in either direction through the print engine. The present invention meets these and other needs in the art.
It is an aspect of the present invention to print images on cut-sheet print media using a digital print engine.
It is another aspect of the invention to expose both the leading and trailing edges of the print media to accomplish xe2x80x9cfull bleedxe2x80x9d printing of the entire sheet of print media.
Another aspect of the invention is to expose the print media as it is moved either forward or backward through the print engine.
A further aspect of the invention is to start print media movement using a first set of pinch rollers, then transfer the movement to a second set of pinch rollers.
The above and other aspects of the invention are accomplished in a printer that feeds cut-sheet print media past an exposure area of a print engine to allow the media to be completely exposed up to the edge of each sheet of the print media.
A sheet of print media is fed into a pair of open leading edge pinch rollers as printing is started. The leading edge pinch rollers then close and push the print media past the exposure area where the leading edge of the sheet is exposed to the image being printed as the media moves past the exposure area. As the leading edge of the sheet exits the exposure area, it passes through a pair of open trailing edge pinch rollers, which close to start pulling the media through the exposure area. After the trailing edge pinch rollers close, the leading edge pinch rollers open, transferring the movement from the leading edge pinch rollers to the trailing edge pinch rollers, allowing the trailing edge pinch rollers to pull the trailing edge of the sheet through the exposure area where it is exposed to the image being printed. This allows both the leading and trailing edges of the sheet of print media to be exposed, thus accomplishing xe2x80x9cfull bleedxe2x80x9d printing of the edges of the sheet of print media.